Existing landing aid systems, enabling autonomous craft to land, are currently based on various technologies.
It is possible firstly to use systems type-approved by civil aviation such as instrument landing systems (ILS) or microwave landing systems (MLS), and their military equivalents, such as PAR-type radars. Such systems are unlikely to be deployed rapidly on a landing site because they require relatively large infrastructures to be put in place on the ground. They are therefore ill-suited to the use and recovery of drones.
Another means of guiding the landing of an aircraft consists in using GPS (or differential GPS) means-based systems which offer the advantage of being inexpensive to implement. However, this solution poses the problem of the availability or the continuity of GPS service in high-accuracy mode. Furthermore, the vulnerability of the GPS systems in the presence of scramblers is well known.
Another means consists in using laser-based telemetry and guidance systems which present the advantage of not requiring any major implementation logistics, but the use of which presents the drawback of being dependent on the weather conditions and the implementation of which entails a phase of scanning for the object to be guided, because of the narrowness of the transmitted beam. Furthermore, additional equipment for absolute positioning relative to the runway is necessary.
Yet another means, similar in principle to the preceding means, consists in using a highly directional, radar tracking-type system, typically operating in the millimetric band. This type of system is, however, sophisticated and therefore costly. Also, like the laser telemetry systems, they too require a scanning phase for the designation of targets and absolute positioning relative to the runway. They are, moreover, sensitive both to climatic conditions and to the configuration of the terrain which constitutes the approach zone of the terrain on which the guided object needs to land. In particular, in the case where several aircraft are to be guided, it is essential to apply timesharing and to proceed by homing from target to target at the risk of losing a target and having to do a complete acquisition of the context. In the approach phase, after all, the guidance constraints to keep the target in the radar beam are very significant.
These last two means are the easiest to deploy on a non-equipped landing field, so they constitute the most commonly used guidance systems. However, with regard to narrow-beam transmission systems that work by pointing to the target from the ground, their implementation entails a scanning phase and dynamic locking onto the target. The risks of stalling and therefore of interrupting the guidance are significant. When the link is critical, an interruption of the tracking can result in erratic guidance with consequences that can be fatal for the aircraft. Furthermore, these solutions are generally costly and have limitations concerning in particular the servo-controlling of the transmitted beam on the position of the target in its approach movement, mechanical limitations that originate in the servo-controls. Also, regarding detection systems servo-controlled on a particular target, these systems are ill-suited to the simultaneous guidance of several drones approaching a landing zone.